In a motor drive VTC shown in JP-U-4-105906A, a control circuit generates a control signal which the driving circuit receives. The driving circuit supplies a current to the motor according to the control signal. The control signal represents a target rotation speed of the motor, which is referred to as the target number hereinafter. The driving circuit applies the current to the motor in such a manner that an actual rotation speed of the motor becomes the target number.
The control signal has a frequency which is proportional to the target number in order to transmit the control signal to the driving circuit correctly.
If the signal line is broken and the control signal is not transmitted from the control circuit to the driving circuit, the driving circuit effectively receives a zero frequency signal. The driving circuit therefore supplies current to the motor as if the frequency of the control signal is zero. In such a case, as the rotation speed of the motor is higher before the signal line break, a rapid change of the rotation speed of the motor occurs so that a rotational-phase changing mechanism may be damaged.
When high frequency noise is superposed on the control signal, the frequency of the control signal may represent a higher rotation speed than the target number. The motor in such a case rotates at a higher rotation speed than the target number so that the rotational-phase changing mechanism and/or the motor may be damaged.